---
title: "Ductile and brittle stress-strain behaviour"
subject: "Material Science"
chapter: 3
key: "semester_3/material_science:ductile-vs-brittle-stress-strain"
source: https://www.bctengineer.com/q/semester_3/material_science/ductile-vs-brittle-stress-strain
site: BCT Engineering
---

# Ductile and brittle stress-strain behaviour

Material Science · Chapter 3: Mechanical Properties and their tests

- Asked in: year not given, 5 marks
- Video: [Ductile vs Brittle Materials: Too Good(USE HEADPHONE)](https://www.youtube.com/watch?v=GyIf_E8vxJw) — Yantriki: The power of Machines
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/ductile-vs-brittle-stress-strain
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-3

## Question

Differentiate between ductile and brittle materials using their stress-strain curves. How is the yield strength determined for a material that has no clear yield point?

## Short answer

Ductile materials undergo large plastic strain before fracture with necking, while brittle materials fracture suddenly with little plasticity. For materials without a sharp yield point, yield strength is determined by the 0.2% offset method: draw a line parallel to the elastic portion from 0.002 strain on the axis, and the intersection stress is the yield strength.

## Answer

A **ductile** material undergoes large plastic deformation before fracture; a **brittle** material fractures with little or no plastic deformation.

```text
 stress
   |    ductile           brittle
   |     ,--.              /
   |   ,'    `.           /
   |  /        x         /
   | /                  x
   |/                  /
   +------------------------> strain
```

| Basis | Ductile material | Brittle material |
|---|---|---|
| Plastic strain before fracture | Large (over 5% elongation) | Very small (under 5%) |
| Yield point | Present (or defined by offset) | Usually absent |
| Fracture | Preceded by necking; cup-and-cone | Sudden, flat fracture, no necking |
| Area under curve (toughness) | Large | Small |
| Examples | Mild steel, copper, aluminium | Cast iron, glass, ceramics, concrete |
| Strength in tension and compression | Nearly equal | Compression much higher |
| Failure warning | Gives warning | No warning |

#### Offset yield strength
Aluminium, copper and high-strength steels show no sharp yield point. The yield strength is then taken by the **0.2% offset method**:

1. Mark a strain of 0.002 on the strain axis.
2. Draw a line from this point parallel to the straight elastic portion.
3. The stress at which this line cuts the curve is the 0.2% proof stress (yield strength).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a brittle material fracture?
   A. With necking
   B. Suddenly with a flat fracture and no warning
   C. By cup-and-cone
   D. After large plastic strain
   Answer: B. Brittle materials fracture suddenly with little plastic deformation. (Key point: Brittle fracture)

2. How is the 0.2% offset yield strength found?
   A. Take the stress at 0.2% strain directly
   B. Line from strain 0.002 parallel to the elastic line; stress where it meets the curve
   C. Take the UTS minus 0.2%
   D. Line from origin at 0.2% slope
   Answer: B. Draw a line parallel to the elastic portion from 0.002 strain. (Key point: Offset method)

3. Which is true about brittle material strength?
   A. Compression strength much higher than tension
   B. Equal in tension and compression
   C. Higher in tension
   D. Not measurable
   Answer: A. Brittle materials are much stronger in compression. (Key point: Strength comparison)

4. Which has larger area under the stress-strain curve?
   A. Brittle
   B. Neither
   C. Ductile
   D. Both equal
   Answer: C. Ductile materials absorb more energy before fracture. (Key point: Toughness)

## Related in Material Science

- [True stress and true strain relations](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Atrue-stress-strain-relations) — Material Science, chapter 3
- [Tensile test and stress-strain diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Atensile-test-engineering-stress-strain) — Material Science, chapter 3
- [Strain-hardening exponent and true stress](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Astrain-hardening-exponent-numerical) — Material Science, chapter 3
- [Cold working and strain hardening](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Acold-working-and-strain-hardening) — Material Science, chapter 4
- [Classification of engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aclassification-of-engineering-materials) — Material Science, chapter 1
- [Galvanic and stress corrosion](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Agalvanic-and-stress-corrosion) — Material Science, chapter 8

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